A conventional technique for correcting brightness (“relighting”) by illuminating a subject in an image with light from a virtual light source is known (Japanese Patent Laid-Open No. 2010-135996). This technique makes it possible to brighten dark areas such as shadows produced by environment light, which enables the image to be corrected so that a subject present in blocked-up shadows, for example, can be recognized.
In the case of relighting on a facial area, for example, an area having a lower luminance than an average luminance of the overall facial area is extracted as a shadow area and the brightness of the shadow area is increased, and as a result shadows in a facial area can be suppressed without affecting the brightness of other areas.
The method disclosed in Japanese Patent Laid-Open No. 2010-135996 raises the brightness of the shadow area by increasing the gain. However, increasing the gain amplifies noise components in addition to signal components, which causes a drop in the S/N ratio of the corrected area. Japanese Patent Laid-Open No. 2010-135996 does apply a noise reduction filter in order to suppress a drop in the S/N ratio, but the noise reduction process is the application of smoothing, which can cause a loss of details in the image.